A common form of ring laser gyroscope comprises a solid block of material having appropriate thermal and optical properties in which is formed a triangular optical path comprising three straight passages formed in the block and having a mirror at each apex. The three passages communicate with each other and contain a gaseous active lasing medium which is excited to produce a discharge in the gas. The gas discharge causes laser action which results in a first beam of laser radiation passing around the optical path in a clockwise direction and a second beam of laser radiation of the same frequency passing around the optical path in a counter-clockwise direction. The laser gyroscope responds to rotational forces about an axis perpendicular to the plane of the optical path by an increase in the frequency of one beam and a decrease in the frequency of the other, the sense and magnitude of the frequency difference being a measure of the sense and magnitude of the rotational force.
Ring lasers of this type suffer from a problem known as lock-in, wherein the two beams remain at the same frequency under low values of rotational force. Prevention of lock-in usually involves applying a mechanical oscillatory rotation or "dither" upon which any rotational force is superimposed. This technique suffers from all the usual problems of moving mechanical systems.
In order to avoid having to apply dither a different type of ring laser gyroscope has been developed, known as a multioscillator ring laser. This has a square or rectangular optical path and produces two pairs of counter-rotating beams, the two beams circulating in the same direction having opposite circular polarisation. Thus one pair of beams consists of right-circularly polarised beams, one going clockwise and the other counterclockwise around the optical path. Similarly the other pair of beams consists of left-circularly polarised beams, one going clockwise and the other counterclockwise around the optical path.
The four beams are the result of including in the optical path a reciprocal dispersion element and a non-reciprocal dispersion element. A reciprocal element, which may be quartz crystal, provides different phase shifts for right and left-circularly polarised beams. The non-reciprocal element provides different phase shifts for light beams travelling through it in opposite directions. The non-reciprocal effect may be produced by a magnetic field acting on the excited gas discharge region of the laser, to produce frequency-splitting by the Zeeman effect. Although the magnetic field may be produced by either a permanent magnet or a solenoid, it is important that the magnetic field strength should be very closely controlled.
It is an object of the invention to provide a multioscillator ring laser gyroscope in which the magnetic field producing Zeeman frequency-splitting may be accurately controlled.